Ultrasound systems exist today that utilize a variety of techniques for processing ultrasound signals to generate information of interest.
The different techniques have been developed for enhancing the quality of the image information which can be made available for diagnosis.
One of the problems to be solved in diagnostic imaging and also in ultrasound imaging relates to increasing image resolution, eliminating artifacts, shadows, increasing edge detail and suppressing speckle.
One known technique that allows to achieve these results is the so called compound imaging.
Spatial compounding is an imaging technique in which a number of ultrasound images of a given target that have been obtained from multiple vantage points or angles are combined into a single compounded image by combining the data received from each point in the compound image target which has been received from each angle. Examples of spatial compounding may be found in U.S. Pat. Nos. 4,649,927; 4,319,489; and 4,159,462. Real time spatial compound imaging is performed by rapidly acquiring a series of partially overlapping component images or frames from substantially independent spatial directions, utilizing an array transducer to implement electronic beam steering and/or electronic translation of the component frames. The component frames are combined into a compound image by summation, averaging, peak detection, or other combinational means. The acquisition sequence and formation of compound images are repeated continuously at a rate limited by the acquisition frame rate, that is, the time required to acquire the full complement of scanlines over the selected width and depth of imaging.
The compounded image typically shows lower speckle and better specular reflector delineation than conventional ultrasound images from a single viewpoint. Speckle is reduced (i.e. speckle signal to noise ratio is improved) by the square root of N in a compound image with N component frames, provided that the component frames used to create the compound image are substantially independent and are averaged.
Conventional approaches to implementing spatial compounding such as that shown in U.S. Pat. No. 4,649,927 typically use a large FIFO memory buffer to temporarily store the component frames that will be compounded (typically by summing and normalization) to form the final compounded image.
Standard ultrasound images compounding is generally provided using images acquired with different steering angles. Each image leads on a fixed line of sight (LOS) angle step. Resulting composed image shows double side discontinuities due to incomplete areas overlapping. To avoid this it is necessary to reduce the field of view of the output Image or heavily filter it.
As compound images are essentially based on the combination of frames taken at different times, when the object under investigation moves, the resulting compounded image contains artifacts. To reduce such artifacts motion compensation techniques can be used.
Motion estimation in ultrasound imaging is a hard problem due to the characteristics of ultrasound images that suffer from poor signal to noise ratio and to possible presence of large flat regions especially in the deep regions. In addition, the local difference between subsequent frames is normally very high, making the motion estimation even more difficult.
Several yes of motion compensation techniques are available in the prior art. One example is the classic block matching estimation. Classical approaches to motion estimation, however, do not prove to be satisfactory in ultrasound compound imaging when motion between frames (e.g., due to moving targets or to the movement of the probe) exist.